1. Field of Invention
The invention relates generally to methods and apparatuses for modifying coherent photonic beams and their intensity profiles. More specifically, the invention relates to such methods and apparatuses that employ a spatial filter to modify photonic beams such as those generated from a laser cavity that is unstable in at least one direction.
2. Description of Background Art
Laser technology is currently being developed to carry out certain thermal processes associated with the fabrication of semiconductor-based microelectronic devices such as processors, memories and other integrated circuits (ICs). For example, the source/drain parts of transistors may be formed by exposing regions of a silicon wafer to electrostatically accelerated dopants containing either boron, phosphorous or arsenic atoms. After implantation, the dopants are largely interstitial, do not form part of the silicon crystal lattice, and are electrically inactive. Activation of these dopants may be achieved by annealing the substrate, e.g., heating the entirety or a portion of the substrate to a particular processing temperature for a period of time sufficient for the crystal lattice to incorporate the impurity atoms into its structure.
In general, it is desirable to anneal semiconductor substrates in a manner that produces well-defined shallow doped regions with very high conductivity. This may be done by rapidly heating the wafer to temperatures near the semiconductor melting point to incorporate dopants at substitutional lattice sites, and then rapidly cooling the wafer to “freeze” the dopants in place. The rapid heating and cooling results in an abrupt change in dopant atom concentration with depth as defined by the implant process. Laser-based technologies are often preferred over conventional heat lamp technologies for annealing because the time scales associated with laser-based technologies are much shorter than those associated with conventional lamps. As a result, thermal diffusion for laser-based annealing processes plays a lesser role in the diffusion of the impurity atoms through the lattice structure than for convention Rapid Thermal Annealing (RTP) technologies employing conventional lamps to heat the wafer surface.
Exemplary terminology used to describe laser-based thermal processing techniques include laser thermal processing (LTP), laser thermal annealing (LTA), and laser spike annealing (LSA). In some instances, these terms can be used interchangeably. In any case, these techniques typically involve forming a laser beam into a long, thin image, which in turn is scanned across a surface to be heated, e.g., an upper surface of a semiconductor wafer. For example, a 0.1-mm wide beam may be raster scanned over a semiconductor wafer surface at 100 mm/s to produce a 1-millisecond dwell time for the heating cycle. A typical maximum temperature during this heating cycle might be 1350° C. Within the dwell time needed to bring the wafer surface up to the maximum temperature, a layer only about 100 to about 200 micrometers below the surface region is heated. Consequently, the bulk of the millimeter thick wafer serves to cool the surface almost as quickly as it was heated once the laser beam is past.
Additional information regarding laser-based processing apparatuses and methods can be found in U.S. Pat. No. 6,747,245 and U.S. Patent Application Publication Nos. 2004/0188396, 2004/0173585, 2005/0067384, and 2005/0103998 each to Talwar et al.
LTP may employ either pulsed or continuous radiation from any of a number of sources. For example, conventional LTP may use a continuous, high-power, CO2 laser beam, which is raster scanned over the wafer surface such that all regions of the surface are exposed to at least one pass of the heating beam. Similarly, a continuous radiation source in the form of laser diodes may be used in combination with a continuous scanning system. Such laser diodes are described in U.S. Pat. No. 6,531,681, entitled “Apparatus Having Line Source of Radiant Energy for Exposing a Substrate”, which was issued Mar. 11, 2003 and is assigned to the same assignee as this application.
In general, illumination uniformity (both macro- and micro-uniformity) over the useable portion of the laser beam image is a highly desirable trait. This ensures that the corresponding heating of the substrate is correspondingly uniform. Similarly, the energy delivered from the laser should be generally stable over time, e.g., energy per pulse for pulse radiation applications and laser beam power for continuous radiation applications, so that all exposed regions are successively heated to a uniform temperature. In short, illumination uniformity and stability is generally a desirable characteristic for any laser used for semiconductor annealing applications.
Nevertheless, illumination stability of a laser may vary depending on the laser's design and construction. All lasers contain an energized substance that can increase the intensity of light passing through it. This amplifying medium contains atoms, molecules or ions, a high proportion of which can store energy that is subsequently released as light. Usually, a transmitted beam is “amplified,” and the amplifying medium is “pumped” to renew its energy supply. Gaseous amplifying media have to be contained in some form of enclosure. Typically, a laser includes a means of pumping an amplifying medium positioned in an optical cavity, i.e. usually between mirrors that extend the optical path in a specific direction. While the mirrors may be arranged in any of a number of different configurations, the slab configuration, in which planar, parallel mirrors are provided, is often used when CO2 is employed as an amplifying medium. The beam within the cavity undergoes multiple reflections between the mirrors and is amplified each time it passes through the amplifying medium. The purpose of the mirrors is to provide directional feedback.
The laser cavity has several important functions. Following pumping, spontaneous emission of light from excited atoms within the amplifying medium initiates the emission of low intensity light into the laser cavity. This light is increased in intensity by multiple passes through the amplifying medium so that it rapidly builds up into an intense beam along an axis defined by the mirrors. In the absence of cavity mirrors, this preferential direction for the emitted light, would not occur. In addition, the cavity ensures that the divergence of the beam is small.
Usually light amplification occurs within a narrow range of wavelengths depending on the amplifying medium and the properties of the optical cavity in which it is contained. Each of the possible stable modes of oscillation is referred to as a cavity mode. Spatial distributions and wavelengths which do not belong to one or a combination of these special modes of oscillation are rapidly attenuated and will not be present in the output beam.
Powerful gas lasers may have an unstable cavity in at least one direction, e.g., in the lateral direction normal to the applied excitation field. For example, multi-kilowatt CO2 slab lasers with diffusion cooling and RF excitation usually have an unstable cavity in the long slab direction and a stable cavity in the short direction of the slab. Such cavities are called hybrid resonators and are optimized in such a way that they produce the lowest order mode, which has the lowest beam divergence. The lowest order mode of an unstable resonator results in a beam with a “top hat” profile with a deep modulation caused by diffraction effects. Improved beam utilization efficiency for semiconductor applications can be achieved by modifying this beam intensity profile to one more closely resembling a square-wave profile with uniform distribution of intensity.
Thus, opportunities exist in the art to improve the performance of lasers and LTP techniques to overcome the drawbacks associated with known technologies for semiconductor annealing applications.